This invention relates to motor speed control systems, and in particular, means for controlling the speed of a sewing machine motor.
In a sewing machine, when the speed thereof becomes excessive, the quality of the stitches formed may deteriorate. Therefore, it is highly desirable to be able to control and limit the maximum attainable speed of the sewing machine. More to this point, with the advent of multitype stitch sewing machines, there may be forms of stitching which require a lower maximum speed than the others. This requirement has led to sewing machine motor control systems which selectively provide two speed ranges over which the sewing machine may operate. U.S. Pat. No. 3,789,783 to Cook et al, discloses an electronic speed regulating system having two speed ranges for the sewing machine motor. These speed ranges are achieved by inserting two fixed resistors in a series in the silicon controlled rectifier gate/motor controller circuit and by selectively bridging one of the resistors. However, in any attempt to adjust the maximum speed in either speed range, the drawbacks of this arrangement become apparent, namely, the difficulty of physically changing the resistors and the fact that the value of one is dependent upon the value of the other.